1. Field of the Invention
This invention relates to the field of electrically powered locks as typically found in security systems.
2. Prior Art
In the security industry, there is a class of electromagnetically actuated locks which are very difficult to power. These locks draw an enormous surge of power to actuate the bolt, but once actuated, draw a much smaller "holding current". These type of locks may be referred to as "surge locks".
Surge locks are often found on doors with big, heavy lock hardware such as crash bars and the like, made by Von Duprin and others. These locks are typically powered by 24 Volts DC, and have two solenoids or "coils": a high power starter coil and a smaller holding coil. The holding coil is energized continuously while the lock is unlocked, but only pulls with enough force to maintain an already retracted bolt in the retracted position. The starter coil pulls with a great deal of force to overcome friction and retract the bolt, but this coil can only be energized for a moment without overheating and burning out. In a good design, the start coil is equipped with a cutout switch which automatically removes power once the bolt is retracted, together with a simple timing circuit which disables the starter coil approximately 300 milliseconds after the first application of power to prevent coil burnout in the event the lock fails to timely actuate for some reason. A surge lock is directly analogous to a split-phase electric motor with separate starting and running windings. In such a motor, the starting winding is shut off by a centrifugal switch after the motor has partially come up to speed. By way of example, surge locks of the foregoing type include the Von Duprin model 33 locks.
Unfortunately, some of the more popular locks are equipped with only the passive timing device. This timing device requires that power be applied suddenly to fire the start coil. The timing circuit used requires that the application of power (24 VDC) have a rapid rise-time in order to trigger properly. If power is applied gradually so that the voltage rises from 0 to 24 Volts DC over a period of 1 second or more, the timing circuit fails to trigger, the lock fails to actuate and is then "hung". A lock in this state has the full 24 Volts applied, but is still locked.
Surge locks draw upwards of 400 watts of power during the start phase (24 VDC at 16 Amps), dropping after 300 ms or so to 1/2 amp of holding current (12 watts). The cost of providing a power supply for these locks which is capable of providing more than 16 amps of continuous current is unreasonable. Also when powered by such a power supply, such a lock may hang up because of an AC power line voltage sag, which of course may originate from causes totally independent of the power supply or lock system operation. For the past several years the assignee of the present invention has tried to address this problem by developing products which attempt to solve this problem more elegantly than by the brute force approach of using a 400 watt power supply to power a 12 watt lock. To do this, the large surge capacity of a small lead-acid storage battery has been used to start these locks. These designs are deceptively simple, and under ideal conditions, they work fine. However, in practice these designs have been less than ideal in the varying conditions found in the field.
In an attempt to solve this problem many designs have been employed. Attempts have been made to provide the required surge current by means of huge capacitors, which proved hopeless. A design is being produced which utilizes dual relays. One relay is actuated continuously and provides a resistively limited holding current. The other relay provides an unlimited surge current, but is only actuated momentarily, by means of a passive capacitor delay circuit. The two relay solution only works with certain types of locks and has problems powering certain types of magnetic locks.
A recent design employs an AC power supply providing regulated and current limited 28 Volts DC and two lead-acid (gel-cell) storage batteries. One battery (the system battery) provides AC power-fail backup for the steady-state load and the other (the surge battery) provides only surge power for starting surge locks. The two types of loads, however, are completely isolated from each other. This results in suboptimal performance when AC power fails. The system battery discharges completely (because of the steady load) while the surge battery remains almost fully charged, but is unable to assist in holding up the steady state load.
These designs exhibit other major problems. When AC power fails, the batteries are allowed to overdischarge completely (to 0 Volts). Overdischarging a lead-acid battery causes plate damage and sulfation which permanently reduce the battery's storage capacity and leads ultimately to shorted cells and early failure. Furthermore, allowing the voltage, under discharge conditions, to fall below 12 Volts serves no useful purpose and can, in fact, cause problems for the equipment being powered. Overdischarging also causes a battery's internal impedance to rise which reduces its ability to accept a charging current, thus, it takes longer to recharge.
When AC power is reapplied with discharged batteries, other problems appear:
1) If the system was set up to energize the lock immediately upon application of power, the battery would never charge fully because the presence of the lock load reduces the charging voltage. Charging voltage must be controlled very accurately. A ten percent change can mean the difference between overcharging and not charging at all. Overcharging will rapidly dry out the battery's electrolyte resulting in a ruined battery.
2) Under weak battery conditions, the system will have insufficient energy to actuate the lock. This places a surge lock in the "hung" state described above, with power applied, but the bolt unretracted. Removing and reapplying the drive signal after the battery has recharged sufficiently to actuate the lock is the only way to clear this condition. Further, as noted above, it is generally necessary to remove the lock electrical load from the battery in order for the battery to recharge.
3) Some designs employed two batteries--one to provide the surge current and the other to provide the holding current. In these designs it is very difficult to get the batteries to charge at equal rates. The charging voltages were, necessarily, slightly different, causing one battery to slightly overcharge, while the other could never quite reach full charge.